May 29 (Thomson Reuters Foundation) - Millions of Afghans face hunger after a drought decimated crops in the war-ravaged country, U.N. officials said on Tuesday, calling for an extra $115 million to help families buy food.

Two thirds of Afghanistan's 34 provinces have been hit by a lack of rain or snowfall since late last year, said a bulletin from the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).

Some rivers and water points have totally dried up, and the last wheat harvest has been "completely lost", according to the Food and Agriculture Organization.

"Six months down the road, millions of people could be in a situation of untenable hunger without knowing where their next meal will come from," said Toby Lanzer, the U.N. humanitarian coordinator in Afghanistan.

Already, the drought has forced 21,000 people to leave their homes and settle on the outskirts of the western city of Herat, said OCHA.

The U.N. hopes to distribute cash to enable families to buy food and try to prevent further migration.

"People prefer cash, which allows them to buy what they need most," he said. "We prefer not to truck food across the country, also because doing so is expensive and can disrupt markets."

The U.N. is revising its humanitarian appeal for 2018 because of the drought, and says it needs an extra $115 million to help 1.4 million of the hardest-hit people.

Only one quarter of the $430 million the U.N. requested for 2018 was funded as of May 23.

The drought has also hurt nomadic herders known as Kuchis, as pasturelands have dried up in some areas, including the northeastern provinces of Badakhshan and Kunduz.

"The animals are too weak to walk to their usual pastures in Badakhshan province and herders have to rent trucks to bring them there," OCHA quoted Abdul Majidi, head of the Kunduz agricultural department, as saying.

Prices for sheep have fallen by as much as 40 percent, but many Kuchis are selling them out of desperation, according to OCHA's report.

The Afghan government and its allies are fighting an array of militant groups, including the Taliban and the Islamic State, which has exacerbated the effects of the drought in some provinces.

"In Helmand, village elders reportedly need to obtain special approval from the armed groups controlling their districts to access markets in areas under government control," said the bulletin.

The cost of wheat shot up by 50 percent and produce prices quadrupled in Kandahar City when roads were temporarily closed in April due to fighting, it said.